Composite anodes



Jan. 20, 1970 a. BlANcHi ETAL COMPOSITE ANODES 3 Sheets-Sheet 1 FiledJan. 16, 1969 Jan. 20, 1970 G. BIANCHI ETAL COMPOSITE ANODES 3Sheets-Sheet 5 Filed Jan. 16, 1969 12 3 35. uzo .8 555: .firue E 315m :0

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VENTORS BIAN cm GALVLONE NEY J (SHN) 110A United States Patent Int. Cl.B01k 3/06 US. Cl. 204242 Claims ABSTRACT OF THE DISCLOSURE Compositeanodes having a metal base selected from the group consisting ofmagnetite, lead dioxide, manganese dioxide and high silicon-iron alloy,preferably containing molybdenum or chromium and having an outer layerof a platinum group metal or a ceramic, semi-conductor coating, whichanodes are useful in electrolytic cells, especially for the productionof chlorine.

PRIOR APPLICATION The present application is a continuation-in-part ofour copending, commonly assigned US. patent application Ser. No.555,026, filed June 3, 1966, now abandoned.

STATE OF THE ART Chlorine is produced in electrolytic cells by theelectrolysis of an aqueous solution of sodium chloride (brine). Chlorineis released at the anode and sodium is released at the cathode andconverted into sodium hydroxide.

Anodes are usually of graphite since it is relatively inexpensive andeasy to shape. However, graphite is rather rapidly consumed during theelectrolysis process (7.5 pounds for each ton of chlorine), requiringfrequent replacement of anodes in order to maintain the desiredelectrode gap with time consuming dismantling and reassemblying of thecells being necessary.

Anodes of platinum group metals, such as platinum, iridium, rhodium,ruthenium, palladium or alloys of these metals, either as such or on asupport have also been used. One particular type of support is titaniumor tantalum. These anodes are more advantageous than graphite, in thatthey are not consumed as rapidly as graphite and the desired electrodegap is maintained over longer periods. They are thus dimensionally morestable and do not require as frequent replacement.

The platinum type anodes, however, have the disadvantage that after ashort period of use, the anode potential increases which causes anincreased energy consumption in the electrolytic process. In addition,although paltinum anodes are not consumed as rapidly as graphite, due tothe high cost of platinum type metals, the depletion at the anode isstill sufficient to add a substantial expense to the operation of thecell.

Various other types of anodes have been proposed such as anodes of aniron or steel base coated with magnetite to prevent erosion. However,due to the difference in the coefiicient of expansion of the base andthe coating, the coating flakes off exposing the iron base which is thenrapidly depleted.

OBJECTS OF THE INVENTION It is an object of this invention to provide ananode which is relatively inexpensive and is not materially consumed inthe electrolytic process and is, therefore, dimensionally stable.

Another object is to provide an anode having good 3,491,014 PatentedJan. 20, 1970 electrical properties which does not have a largepotential increase with time and use.

It is a further object of this invention to provide an anode having thedesirable stability and other characteristics of an iron base and aplatinum surface without the disadvantages of prior anodes.

These and other objects and advantages of the inven tion will becomeobvious from the following detailed description.

THE INVENTION The electrodes of the invention which achieve the aboveobjects and overcome the disadvantages of the prior art are comprised ofa metal base selected from the group consisting of mangetite, manganesesdioxide, lead dioxide and high silicon-iron having a silicon content ofabout 14 to 16% which may have alloyed with it about 3 to 4% of eithermolybdenum or chromium or other metals, preferably from the sixth groupof the periodic table, the remainder being iron, which base is coatedwith a thin layer of platinum or a platinum group metal or alloys of theplatinum group metals or a ceramic, semi-conductor coating.

The high intrinsic corrosion resistance of high Si-Fe alloys (14.5 Si)has long been known. The most significant discovery relating to theaddition of other elements was the profound effect of 3% molybdenum onthe resistance of these alloys to hot hydrochloric acid and variouschlorides. This alloy is described in US. Patent No. 1,972,103. It isfurther known that extra resistance to hot hydrochloric acid is attainedby adding 3 to 4% M0 instead of raising the silicon content beyond about14 to 16%. It is thus possible to avoid the extreme fragilitycharacteristic of the 17% Si-Fe alloy, as would otherwise be requiredfor hot HCl. A relatively recent discovery is that chromium can be usedas a constituent of high siliconiron, in the place of molybdenum. It isalso known that a 14.5% Si, 3.5% Mo-Fe alloy has good behavior incontact with free, wet chlorine.

The improved behavior of the high silicon-iron alloy includingmolybdenum has been recognized also in its use as anode material forcathodic protection in sea water. However, we have now also found thatunder such conditions a slight amount of noble metal of the platinumfamily is suflicient to reduce the anodic corrosion rate to negligibleamounts. Indeed, just as in the case of titanium, even a partialcoverage with a noble metal allows the silicon-iron substratum toacquire and keep passivation also under chlorine discharge, providedthat the current density is not so high as to raise the anodic potentialbeyond a trans-passivity limit. It has now been found that inconcentrated sodium chloride solution, the trans-passivity limit for the15% Si-3% Cr-Fe alloy is about 1.8 v.

The high silicon-iron alloys can be used for electrolytic cell anodes inthe chlor-alkali industry as a passive support for a thin film of noblemetal of the platinum group, such as platinum, iridium, ruthenium,rhodium and palladium or alloys thereof.

The anodes are useful in various types of electrolytic cells. However,the molybedenum and chromium content alloys are more suitable for use indiaphragm cells because of the possibility of catalytic evolution ofhydrogen with these metals in mercury cells. The anodes are also usefulfor cathodic protection of underwater metal objects such as ships,piers, etc.

The metal base anodes of the invention provided with a ceramicsemi-conductor coating, have a semi-conductive mixed metal oxide coatingover part or all the metal base sufficient to conduct an electrolysiscurrent from the base to an electrolyte over long periods of timewithout passivation or increase in overvoltage. The mixed metal oxide iscomprised of titanium oxide or doped titanium oxide or tantalum oxide ordoped tantalum oxide or mixed metal oxides from adjacent groups in theperiodic table.

It has long been known that rutile or titanium dioxide and tantalumoxide have semi-conducting properties, either when doped with traces ofother elements or compounds which disturb the lattice structure andchange the conductivity of the titanium dioxide or tantalum oxide, or-

when the lattice is disturbed by the removal of oxygen from the titaniumdioxide or tantalum oxide crystal. Titanium dioxide has been doped withtantalum, niobium, chromium, vanadium, tin, nickel and iron oxides andother materials to change the electrical conducting or thesemi-conducting properties of the titanium dioxide, and by changing thestoichiometric balance by removing oxygen from the crystal lattice.Likewise, Ta O films have had their conductivity altered by ultravioletradiation and by other methods, but no one has suggested the use ofdoped titanium dioxide or tantalum oxide to provide a conductive orsemi-conductive face on the base metal electrode for use inelectrochemical reactions. Other metal oxides when intimately mixed andheated together have the property of forming semi-conductors,particularly mixed oxides of metals belonging to adjacent groups in thePeriodic Table.

Various theories have been advanced to explain the conductive orsemi-conductive properties of doped or undoped titanium dioxide, alsofor Ta O See, for example, Grant, Review of Modern Physics, vol. 1, page646 (1959); Frederikse, Journal of Applied Physics, Supplement to vol.32, No. 10, page 221 (1961) and Vermilyea, Journal of theElectrochemical Society, vol. 104, page 212 (1957), but there appears tobe no general agreement as to what gives doped titanium dioxide andtantalum oxide their properties of semi-conduction. When other mixedmetal oxides are used to produce semi-conductors, it is possible thatoxides of one metal belonging to an adjacent group in the Periodic Tablepenetrates into the crystal lattice of the other metal oxide by solidsolution to act as a doping oxide which disturbs the stoichiometricstructure of the crystals of one of the metal oxides to give the mixedoxide coating its semi-conducting properties.

In general, it is preferred to make a solution of the semi-conductormetal and the doping composition in such form that when applied andbaked on the cleaned base metal electrode the solution will form TiOplus doping oxide or Ta O plus doping oxide or other metal oxide plusdoping oxide and to bake this composition on the base metal electrode inmultiple layers so as to form a solid solution of the TiO T a O or othermetal oxide and the doping oxide on the face of the electrode which willhave the desired semi-conducting properties and will continue chlorinedischarge without increase in overvoltage over long periods of time. Anysolutions or compounds which on baking will form TiO plus doping oxide,Ta O plus doping oxide or otherv metal oxide plus doping oxide may beused, such as, chlorides, nitrates, sulfides, etc., and the solutionsgiven below are only by way of example.

Overvoltage as used above may be defined as the voltage in excess of thereversible or equilibrium which must be applied to cause the electrodereaction to take place at the desired rate. Chlorine overvoltage varieswith the anode material and its physical condition. It increases withanode current density but decreases with increase in temperature.

Titanium dioxide, tantalum oxide and other metal oxide semi-conductorfaces may be produced by doping titanium dioxide, tantalum oxide orother metal oxide crystals with various doping compositions or bydisturbing the stoichiometric lattice by removing oxygen therefrom tocause the TiO Ta O or other metal oxides to become semiconductive.Because of the tendency of the TiO Ta O or other metal oxide crystals tobecome reoxidized, it is preferred to form the semi-conductive faces onour elec- 4 trodes by the use of doping compositions which in bakingform solid solutions with the TiO Ta O or other metal oxide crystalswhich are more resistant to change during electrolysis processes.However, semi-conducting coatings produced by withdrawing oxygen fromthe T iO Ta O or other oxide lattices to cause lattice defects ordeficiencies may be used on the electrodes of the invention.

Various doping materials which introduce impurities into the TiO and TaO crystals to make them semiconductive, may be used to increase theconductivity and electrocatalytic properties of the TiO and Ta O layer011 the electrode, W02, P205, Sb O5, V205, T3205, Nb205, B203, Cr2O B60,N320, CaO, SrO, R1102, 1102, PbOg, OsO PtO AuO AgO SnO A1 0 and mixturesthereof. The best results have been secured with doping compositions forTiO which have the tetragonal rutiletype structure with similar unitcell dimensions and approximately the same cationic radii (0.68 A.).Thus, RuO (0.65 A.) and IrO (0.66 A.) are especially suitable dopingcompositions as well as other oxides of metals of the platinum group(i.e., platinum, palladium, osmium and rhodium). IrO forms solidsolutions in TiO up to about 5 mole percent IrO when heated together at1040 C. At lower temperatures, the amount of H0 which will form solidsolutions in TiO is lower but the amount of platinum metal oxide groupwhich is not in solid solution continues to act as a catalyst forchlorine discharge.

Oxides of metals from Group VIII of the Periodic Table of elements aswell as oxides of metals of Group VB, Group VI-B, oxides of metals fromGroup I-B and oxides of elements from Group V-A, as well as mixtures ofthese oxides capable on baking of forming solid solution crystals withTiO and Ta O and of interrupting the crystal lattice of Ti0 and Ta O maybe used to form semi-conductor and electrocatalytic coatings on the basemetal electrodes.

In forming semi-conductor coatings for base metal electrodes from othermetal oxides, it is preferable to use mixed oxides of metals, ormaterials which form mixed oxides of metals, from adjacent groups of thePeriodic Table, such as, for example, iron and rhenium; titanium,tantalum and vanadium; titanium and lanthanum. Other oxides which may beused are manganese and tin; molybdenum and iron; cobalt and antimony;rhenium and manganese and other metal oxide compositions.

The percentage of the doping compositions may vary from 0.10 to 50% ofthe TiO Ta O or other metal oxide and surprising increases inconductivity of the TiO Ta O or other metal oxide facing can be gottenwith as little as 0.25 to 1% of the doping composition to the TiO Ta Oor other metal oxide in the conductor face on the electrode. It ispreferred, however, to use sufiicient excess of the doping metal oxideto provide a coating on the anodes which will catalyze chlorinedischarge without material overvoltage.

Examples of suitable ceramic semi-conductive mixed metal oxides are54.8% TiO 22.6% IrO and 22.6% RuO; 10% RuO, 10% IrO and Ta O 15% RuO, 5%AuO and 80% TiO 35% RuO, 55% TiO and 10% SnO; 45% RuO, 54% TiO and 1% A10 45% RuO, 50% TiO and 5% Ta O etc.

The conductive coating of the invention may be applied in various ways,and to various forms of base anodes, such as solid rolled massiveplates, perforated plates, slitted, reticulated plates, mesh and rolledmesh, woven wire or screen, rods and bars or similar metal plates andshapes. The preferred method of application is by chemideposition in theform of solutions painted, dipped or sprayed on or applied as curtain orelectrostatic spray coatings, baked on the anode base, but other methodsof application, including electrophoretic deposition orelectro-deposition, may be used. Care must be taken that no air bubblesare entrapped in the coating and that the heating temperature is belowthat which causes warping of the base material.

The spectrum of doped Ti samples shows that the foreign ion replaces theTi ion on a regular lattice site and causes a hyperfine splitting inaccordance with the nuclear spin of the substituting element.

In all applications, the metal base is preferably cleaned and free ofoxide or other scale. This cleaning can be done in any way, bymechanical or chemical cleaning, such as, by sand blasting, etching,pickling or the like.

This invention may be further understood from the following detaileddescription of preferred embodiments and by reference to the drawings,in which:

FIG. 1 is a perspective of a diaphragm cell showing location of anodes;

FIGS. 2 and 3 are graphs of anode potential against current;

FIG. 4 is a graph of anode potential against time; and

FIG. 5 is a graph of anode weight loss against time.

The embodiment of a diaphragm cell, illustrated in FIG. 1, has verticalanodes imbedded in a lead base and cathodes separated from the anodes bydiaphragms.

The anodes of this invention may be used in various electrolysisprocesses and the cell shown in FIG. 1 is only for the purpose ofillustrating one application of anodes and process.

The base of the cell of FIG. 1 consists of a shallow cast iron pan 1,housing flat copper grids, not shown. In contact with these grids arelateral rows of anodes 2, which extend vertically from the base 1 andare secured in the base and electrically connected to the copper gridsby having molten lead poured around the base of the anodes and thecopper grids.

The cathode assembly which rests on the base is a rectangular steelshell 3, having an inner section consisting of lateral rows of doublemetal screens 4 upon which a diaphragm of asbestos fiber is deposited.When the cathode assembly 3 is positioned on the base 1, the rows ofscreens 4 fit alternately between the rows of anodes 2 to form theanolyte and catholyte sections of the cell. Readily accessibleelectrical connections for anodes 2 are made by lugs 5 which extend fromthe copper grids in base 1 to a copper grid bar 6 around the outside ofthe cathode assembly 3. The cell assembly is completed by a concretehead 7 on top of the cathode assembly 3. Cells of this type are knowncommercially as D3 cells.

Brine is introduced to the cell by brine feed assembly 8. Cell liquor istaken out through conduit 9, having a standpipe to maintain brine levelin the cell. Chlorine is withdrawn through outlet 10 and hydrogenthrough outlet 11. The anodes 2, instead of the usual graphite, are madeof a platinum coated high silicon-iron composition, preferably about Si,about 3% Mo, about 0.6 to 0.7% carbon and the remainder iron. Otheralloy ingredients, such as chromium, may be used in place of molybdenum.This composition anode may be used in other form of diaphragm cells thanthat illustrated in FIG. 1, and with proper design may be used inmercury cells and for other electrolysis purposes and for corrosioncontrol.

The platinum coating is of the order of about 1 micron in thickness andmay be deposited on the high siliconiron anodes by electroplating,chemical deposition, from a platinum containing solution, spraying orany other method which will produce a substantially uniform andsubstantially non-porous coating. In place of platinum, other platinumgroup metals and alloys thereof, with platinum and with each other, maybe used.

The following specific examples of preparation of the improved anodesand tests thereof are given to enable persons skilled in the art tobetter understand the invention and are not intended to be limitative.

EXAMPLE I Composite anodes were prepared as follows. The two alloys usedfor comparison testing had the following composition:

6 Alloy A: Iron Percent Silicon 15 Alloy B: Iron Silicon 15 Chromium 3The samples were shaped as discs 3 cm. in diameter and 0.5 cm. thick,with a central hole 0.8 cm. diameter.

Surface preparation Two alternative methods of surface preparation, onechemical and the other mechanical, were used as follows:

(a) Chemical preparati0n.Acid pickling at 40 C. for 5 minutes in thefollowing solution contained in a polyethylene vessel:

HNO (RP) 86% 10% ml. HF (RP) 10% 10 ml. H O (distilled) 30 ml.

(b) Mechanical surfacing by the grinding wheel, followed by sandblasting-The platinum deposits were made by electroplating and had moreuniform appearance when the substratum was prepared according toalternative (b). Accordingly, all samples subjected to the subsequenttests were prepared by mechanical surfacing only.

Platinum plating.-Bath composition:

The electroplating time was established after determining the currentefiiciency under the above noted conditions, so as to obtain a platinumdeposit of 28.8 g./m. or about 1.25 microns Pt. The current efficiencywas 23 to 25% with reference to the reaction: I

Pt(NH ++[-2e'- Pt+2NH The anodes prepared for testing were as follows:

(A) Fe-Si (B) Fe-Si-Pt c Fe-Si-Cr (D) Fe-Si-Cr-Pt EXAMPLE II Anodicpolarization curves The anodic curves on the samples were determined atan anodic current density of 0.75 a./cm. (4.83 per sq. in.).

The solution was kept at 300 g. NaCl per liter and at 70 C., pH 2 to 3.

The results obtained on the two samples B and D, with and withoutchromium, are illustrated in FIGS. 2 and 3, respectively. Whereas in thepresence of 3% Cr (Sample D, FIG. 3), the overtension has increased byless than 0.1 v. after 816 hours at said current density, theovertension on the sample without chromium (Sample B, FIG. 2) was almost0.3 v. higher than the initial value (1.4 v.) after 744 hours. Thepotential rise in time for both samples is represented in FIG. 4. Nocorrosion phenomena were visible on the chromium alloyed Sample D, butthe other Sample B produced a yellow coloring on the solution, due todissolved iron ions. The corrosion phenomenon which was also checked byanalytical determination, began after 192 hours of operation andsubsided after 336 hours. Thereafter, it became observable again after744 hours of operation.

7 EXAMPLE 111 Chemical corrosion tests The behavior of several samplesin chlorinated brine at 70 C., is shown in FIG. 5, where weight lossesare plotted versus time. The unplatinized Sample E without Cr had by farthe highest corrosion rate (about 4 mg./hr. cm. however on Sample F madeof the same alloy, and platinum coated on one face only, the initialcorrosion rate was less than 0.1 mg./ hr. cm. and tended to becomenegligible after 720 hours.

In alloys G and H, containing chromium, the corrosion rate becamenegligible in about 120 hours, while its initial value (Sample G) waspractically reduced to nil by platinizing one face of the sample.

EXAMPLE IV I This example shows the comparison of the terminal effectexerted by electrode resistance on current density in (1) a graphiteanode, (2) a platinum plated titanium anode and (3) a platinum platedsilicon-iron anode (Anode B in Example I).

The anodes (1) graphite, (2) platinum plated titanium and (3) platinumplated silicon-iron were evaluated in a Type D-2A Cell, basicallysimilar to the cell illustrated in FIG. 1.

Each anode had a working face 69 cm. in length and 15 cm. wide, with theanode-to-cathode distance being about 1 cm. The results are tabulated inTable I. Calculations were made by the method of C. W. Tobias, JournalElectrochemical Society, 100, 459 (1953).

TABLE I.TERMINAL EFFECT FOR DIFFERENT ANODE STRUCTURES IN D-2A CELL Highsilicon- Iitanium iron Graphite (It coated) (Pt coated) 1 (cm) 69 G9 69(1. (cm.) 1 1 1 ll 0. 0145 0. 0145 0. 0145 g (ohm. cm 800Xl- 60 10-90x10" t (thick. cm 3. 18/0. 1 0. 2 1. 2 1 F0 [1' (av; 1. 094/1. 3 1.14 1. 0409 iu-l /1' rm) 0. 95/0. 86 0. 93 0. 98 i (1-o)/i(x-I) 1. 15/1.5 1. 22 1. 06

The values shown in Table I illustrate the ratios of current density iat the bottom edge of the anode to its value i at the upper edge.

These ratios indicate that there is a considerable unevenness in currentdistribution for all the structures considered, with the exception ofthe platinum coated high silicon-iron anode, for which the currentdensity was only 6% higher at the bottom than at the top.

In particular, it can be noted that the unbalance in the case of thegraphite anodes is by itself sufiicient to explain the higherconsumption rate in the lower portion of graphite anodes, aside from thefurther disturbance caused by the blanketing effect of the rising gasbubbles, whereby the current distribution is made to depart even fartherfrom uniformity.

As regards the platinum coated titanium anodes, even when the titaniumstructure is 2 mm. thickness the difference between lower and uppercurrent density values is still as great'as 22%. Such difference is toohigh, especially if the additional effect of the gas blanketing bubbleaction is considered; indeed, this would bring about different rates ofplatinum consumption and passivation at different places along theanodes, with a consequent decrease in the platinum coating life.

In the platinum coated high silicon-iron anodes, the terminal effectcalculated for a 12 mm. thickness, which is recommended for mechanicalsafety, shows that such thickness should not be materially diminished ifthe unevenness in current distribution is to be kept within narrowlimits. A

An economic comparison shows that the initial cost of A magnetite anodeplate,"'with a surface of 50 cm. projected area, was cleaned by boilingat reflux temperature of 110 C. in a 20% solution of hydrochloric acidfor 40 minutes. It was then given a liquid-coating containing thefollowing materials: l

Ruthenium as RuCl -H O mg. (metal)-.. I 10 Iridium as (NH lrCl mg.(metal) 10 Titanium as TiCl ccmg. (metal) 56 Formamide (HCONH c drops"-10-12 Hydrogen peroxide (H 0 30%) do 3-4 The coating was prepared byfirst blending or mixing the ruthenium and iridium salts containing therequired amount of Ru and Ir in a 2 molar solution of hydrochloric acid(5 ml. are suflicient for the above amounts) and allowing the mixture todry at a temperature not higher than 50 C. until a dry precipitate isformed. Formamide is then added to the dry salt mixture at about 40 C.to dissolve the mixture. The titanium chloride, TiCl dissolved inhydrochloric acid (15% strength commercial solution), is added to thedissolved Ru-Ir salt mixture and a few drops of hydrogen peroxide (30% H0 are added, sufiicient to make the solution turn from the blue color ofthe commercial solution of TiCl to an orange color.

This coating mixture was applied to both sides of the cleaned titaniumanode base, by brush, in eight subsequent layers. After applying eachlayer, the anode was heated in an oven under forced air circulation at atemperature between 300 and 350 C. for l0.to 15 minutes, followed byfast natural cooling in air between each of the first seven layers, andafter the eighth layer wasapplied the anode was heated at 450 C. for onehour under forced air circulation and then cooled.

The amounts of the three metals in the. coating correspond to the weightratios of 13.15% Ir, 13.15% Ru and 73.7% Ti and the amount of noblemetalv in the coating corresponds to 0.2 mg. Ir and 0.2 mg. Ru persquare centimeter of projected electrode area. It is believed that theimproved qualities of this anode are due to the fact that although thethree metals in the coating mixture are originally present as chlorides,they are co-deposited on the base in oxide form. Other solutions whichwill deposit the metals in oxide form may, of course, be used.

EXAMPLE VI A magnetite anode plate of the same size as in Example V wassubmitted to the cleaning and etching procedure as described above andthen given a liquid coating containing the following materials:

Ruthenium as RuCl '-'H O mg. (metal) 10 Iridium as IrCl mg. (metal) c10v Tantalum as TaCl -4 mg. (metal) Isopropyl alcohol drops 5Hydrochloric acid (20%) ml 5 The coating Was prepared by first blendingor mixing the ruthenium and iridium salts in 5 ml. of 20% HCl. Thevolume of this solution was then reduced .to about one-fifth by heatingat a temperature of C. The required amount of TaCl was dissolved inboiling 20% HCl so as to form a solution containing about 8% TaCl byweight. The two's'olutions were mixed together and the overall volumereduced to about one-half by heating at 60C. The specified quantity ofisopropyl alcohol was then added.

The coating mixture was applied to both sides of the cleaned anode basein eight subsequentlayers and following the same heating and coolingprocedure between each coat and after the final coat as described inExample V. v

The amounts of the three metals in the coating correspond to the weightratios of Ru, 10% Ir and 80% Ta and the amount of noble metal in thecoating corresponds to 0.2 mg. Ir and 0.2 mg. Ru per square centimeterof projected electrode area.

EXAMPLE VII A manganese dioxide anode plate was submitted to a cleaningand etching procedure and then given a liquid coating containing thefollowing materials:

Mg./cm. (metal) Ruthenium as RuCl -3H O 0.8 Titanium as TiCl 0.89Tantalum as TaCl 0.089

The coating mixture was prepared by first blending the dry rutheniumsalt in the commercial hydrochloric acid solution containing TiClTantalum was then added in the above proportion and in the form of asolution of 50 g./l. TaCl in HCl. The blue color of the solution wasmade to turn from blue to orange by introducing the necessary amount ofhydrogen peroxide, which was followed by an addition of isopropylalcohol as a thickening agent. The coating mixture was applied to bothsides of the anode base by electrostatic spray coating in foursubsequent layers. The number of layers can be varied and it issometimes preferable to apply several coats on the area facing thecathode and only one coat, preferably, the first coat, on the area awayfrom the cathode. After applying each layer, the anode was heated in anoven under forced air circulation at a temperature between 300 and 350C. for 10 to '15 minutes, followed by fast natural cooling in airbetween each of the first three layers and after the fourth layer wasapplied the anode was heated at 450 C. for one hour under forced aircirculation and then cooled.

The amounts of the three metals in the coating correspond to the weightratios of 45% Ru, 50% Ti, 5% Ta.

EXAMPLE VIII The coating mixture consisted of an HCl solution containingthe following salts:

Mg./cm. (metal) Manganese as Mn(NO 0.5

Tin as SnCl -5H O 0.5

The solution was prepared by first blending the two salts in 0.5 ml. of20% HCl for each mg. of overall salt v amount, and then adding 0.5 ml.of formamide. The solution was heated at 40-45 C. until reachingcomplete dissolution, and then applied in six subsequent coatings on thepre-etched base with a thermal treatment after each layer as formerlydescribed.

EXAMPLE 1X EXAMPLE X Using the same procedure as in Example VIH, thefollowing binary mixture was applied to a base electrode:

Mg./cm. (metal) Cobalt as C001 0.5 Antimony as SbCl (C0OI-I) (CHOH) 0.

10 EXAMPLE x1 Four coating types were tested, each of which con-(commercial) 0.7 Lanthanum as La(NO -8H O 0.088 Tin as SnCl -5H O 0.15platinum as PtCl -nH O (commercial) 0.85 Sample No. 2:

Titanium as TiCl in HCl solution (commercial) 0.7 Lanthanum as La(NO -8HO 0.088 Tin as SnCl -5H O 0.15 Rhodium as (NH RhCl 0.85 Sample No. 3:

Titanium as TiCl in HCl solution (commercial) 0.7 Aluminum as AlCl -6H O0.088 Tin as SnCl -5H O 0.15 Iridium as IrCl 0.85 Sample No. 4:

Titanium as TiCl in HCl solution (commercial) 0.7 Aluminum as AlCl -6H O0.088 Tin as SnCl -5H O 0.15 Palladium as PdCl 0.85

While certain specific embodiments and preferred modes of practice havebeen set forth above, it will be recognized that this is mainly for thepurpose of illustrating the invention to persons skilled in the art, andthat various changes and modifications may be made without departingfrom the spirit of the disclosure or the scope of the appended claims.

We claim:

1. An electrode comprising a metal base selected from the groupconsisting of magnetite, manganese dioxide, lead dioxide and highsilicon-iron having a silicon content of 14 to 16% and optionallycontaining a metal from the sixth group of the Periodic Table, said basebeing coated with a thin layer selected from the group consisting of atleast one platinum group metal and a ceramic semi-conductor coating.

2. The electrode of claim 1 wherein the base is high silicon-ironcontaining from about 3 to 4% of a metal selected from the groupconsisting of chromium and molybdenum.

3. The electrode of claim 1 wherein the layer consists of metals andalloys of metals from the platinum group on said base.

4. The electrode of claim 1 wherein the base is high silicon-ironcontaining from 3 to 4% of a metal from the sixth group of the PeriodicTable.

5. The electrode of claim 1 wherein the base is iron containing about 14to 16% of silicon and the layer is a platinum group metal.

6. The electrode of claim 1 wherein the base is iron containing about 14to 16% of silicon and the layer is a ceramic, semi-conductive layer ofmixed metal oxides.

7. The electrode of claim 6 wherein the mixed metal oxides is a dopingmetal oxide and an oxide selected from the group consisting of tantalumoxide and titanium dioxide.

8. The electrode of claim 6 wherein the ceramic, semiconductive layercontains titanium dioxide.

9. The electrode of claim 8 wherein the other metal oxide is a platinummetal oxide.

10. In an electrolytic cell, an anode comprising an iron base containingabout 14 to 16% of silicon and about 3 to 4% of a metal selected fromthe group consisting of chromium and molybdenum and a coating selected9/1934 Parsons 1483 XR 3/1938 Ihrig 29-196 XR 3,376,209 4/1968 Sabins204290 XR JOHN H. MACK, Primary Eraniirle'r W D' R- JORDAN, AssistantExaminer" ""iisfbl. 511R:

